Until quite recently, natural influences, such as periodic shifts in Earth's orbit, effected the changes. But the ecosystem shifts they observe since 1950 indicate that something other than Earth's wobbly orbit is changing the Arctic's climate.

As another recent study pointed out, going strictly by orbital shifts, the Arctic should be cooling at present. Earth is now 0.6 million miles farther from the sun during the northern hemisphere's summer solstice than 2,000 years ago, and receives less of its energy.

And, in fact, the Arctic was gradually cooling for the past 2,000 years — until about 50 years ago when, despite the diminished solar energy reaching the northern hemisphere during summer, it abruptly began warming. (Here's a nice graphic of that trend and its abrupt reversal.)

And the Arctic lake ecosystem appears to have adjusted accordingly.

The authors of the PNAS study examined the relative abundances of midges and algae in the sediments of a lake on Baffin Island, which lies a few hundred miles west of Greenland. For the past several thousand years, cold-adapted midge larvae have abounded in the lake.

Then came 1950. Since then, midge numbers have undergone a steep decline. Two species have disappeared entirely.

In that same period, a lake alga species, which was relatively scarce before the 20th century, has increased in abundance. The authors attribute this to less lake ice. Without it, the photosynthetic alga has more access to sunlight.

This study comes on the heels of a several others documenting other ecosystem shifts in the Arctic. A review last month in Science summed them up. In the past 150 years, Earth's average temperature has increased by about 0.4 C (0.72 F.).

But the Arctic has warmed two to three time as much. As a result, spring arrives earlier and winter is less severe. Some plants begin flowering 20 days earlier than previously. Insects show up earlier. Herbivorous insects, like the winter moth, have moved north, defoliating birch forests in parts of Scandinavia. Red foxes have moved north into Arctic fox territory.

Some migrating caribou herds, meanwhile, find that their calving season is out of sync with the most nutritious forage, which now sprouts earlier. These herds have declined.

Conversely, in some places, snow that now melts more often during winter permits caribou herds to access more food during cold months. These herds have responded by growing in number.

Another recent study in the journal Ecological Monographs looked at the positive feedback potential of a warming Arctic. Arctic land and sea together absorb and sequester up to one-quarter of the world's carbon.

Much of that carbon is held in permafrost. If the Arctic warms too much, it may not only stop absorbing carbon, it may begin emitting greenhouse gases, warming climate further.

Another recent bit of climate news has little to do with climate science directly or observations of shifting ecosystems. Rather, it purports to measure Americans' perception of climate change.

Andrew Kohut, director of the research center, attributed the decline to the economic downturn. "The priority that people give to pollution and environmental concerns and a whole host of other issues is down because of the economy and because of the focus on other things," he told The Associated Press.